There is conventionally known a Synthetic Aperture Radar (SAR) as an active sensor for measuring a reflected wave of an electromagnetic wave (a microwave pulse) radiated to the ground surface. By use of the characteristic of the microwave, the Synthetic Aperture Radar can extensively photograph the ground surface, day and night, regardless of weather conditions. Further, some of the latest satellite-borne Synthetic Aperture Radar can photograph images with a resolution of 1 m. However, due to the influence of noise, complicated mechanism of scattering and the like, it is difficult to interpret terrestrial objects from a radar image (referred to as a “SAR image” hereinafter) photographed from a platform such as an artificial satellite, an aircraft or the like.
On the other hand, since an optical image is taken at visible wavelengths, it is possible to obtain the same information as viewed by the naked human eye from the optical image, and therefore the optical image has the advantages that the terrestrial objects can be easily interpreted. However, the problem with the optical image is that it can be taken only in good weather.
Currently, attempts are being made to precisely grasp terrestrial object information by using a data fusion technique in which the SAR image and the optical image are combined with each other (refer to, for example, “Research Report on Commercialization of Remote Sensing with Synthetic Aperture Radar (Summary)”, System Technology Development Research 17-R-6, March 2006, p. 11-12, The Mechanical Social Systems Foundation (contract researcher: Japan Resources Observation System Organization)).
The SAR image is an image based on reflection intensity of an electromagnetic wave (i.e., an image based on scattering intensity), and is affected by the dielectric constant of the photographic subject. Further, due to image distortions caused by foreshortening, layover, radar shadow and the like, and different photographing times, it has been difficult to synthesize the SAR image and the optical image taken by an optical satellite to create a synthetic image (i.e., a pan-sharpened image), and therefore the practical use of the synthetic image of the SAR image and the optical image has not progressed in the past.
It is believed that concrete reasons why the synthetic image of the SAR image and the optical image has not progressed are as follows. As described above, since the SAR image is affected by the characteristics (the dielectric constant and the like) of the material of the photographic subject and the shape of the photographic subject, generally it has the feature that the building area will have high brightness value (i.e., the building area will be bright) due to high reflection intensity of buildings, while the occlusion areas (shadows) will have low brightness value (i.e., the occlusion areas will be dark) due to low reflection intensity of the shadows. Thus, in the urban area, the brightness value of the pixels corresponding to the buildings is often saturated, and therefore the histogram will have a peak in an area having a high brightness value (i.e., a high-brightness area). On the other hand, shadows tend to occur in the forest area or the like, and therefore the histogram will have a peak in an area having a low brightness value (i.e., a low-brightness area). At this time, if the SAR image and the optical image are used to produce a pan-sharpened image (i.e., a synthetic image) while leaving the brightness value (i.e., the histogram characteristics) as it is, the white pixels or the black pixels will be dominant, and therefore the optical image in the synthetic image will be difficult to be seen. In other words, the information contained in the optical image before the optical image is synthesized will be difficult to be interpreted from the synthetic image.
In view of the aforesaid problems, it is an object of the present invention to produce a synthetic image which enables easy interpretation of the terrestrial objects, using a radar image, such as a SAR image, and an optical image.